The rapid development of the telecommunications industry have made wireless handheld devices like cell phones, pagers, two-way messaging devices, etc. massively popular, creating a need for new electronic components and circuits in both mobile and base station systems as competition drives the introduction of expanded capabilities.
Radio frequency power amplifiers contribute to the power consumption in the communications networks. One reason for this is the complexity of wireless communication networks, with intense baseband signal processing. The major source of power consumption in a base station is the base stations power amplifiers. The high power consumption of the power amplifiers is mainly due to low mean efficiency where a lot of the input power is converted into heat. This requires both costly and space demanding arrangements to allow for sufficient cooling of the system.
Also, the power consumption of the system reflects on other aspects such as battery backup systems and the operators cost of power over the lifetime of the radio base station. All extra means needed to compensate for high power consumption and low efficiency will accumulate on the end customer's price for the base station.
The low efficiency of the power amplifier is due to the amplifier working under backed off conditions for most of the time. This is because a signal according to e.g. WCDMA has a high peak to average ratio. Inherently the efficiency of an amplifier is at its best when the amplifier is working close to saturation. For backed off conditions, the efficiency decays very fast with reduced output power.
Efficiency in back off denotes efficiency when an amplifier is used below its maximum available output power. Back off is the difference in decibels between actual used output power and the maximum available output power of the amplifier.
Thus, to reduce operating costs of base stations and extend battery life in mobile units, there is a need to develop new amplifiers to replace the traditionally inefficient, power wasting, elderly designs currently in use.
Many contemporary base station amplifiers employ complex techniques to realize amplifiers with a high degree of linearity over a broad frequency range. Unfortunately those solutions have a low efficiency when working in power back off.
Handset power amplifiers also suffer from efficiency problems, often more critical than those for base stations as the power supply for mobile user equipment is strictly limited. Today's smaller, faster and more effective portable electronics demand high power with minimized losses.
Load modulation networks have been suggested as feasible means of maintaining the efficiency of a power amplifier at reduced output power. This may be useful when the amplifier works on signals with a high peak to average signal amplitude, for example WCDMA signals.
An important characteristic of a load modulation network is the ratio of input impedance change in relation to the change of the tuning device. This will set the dynamic range of the load modulation network for a given tuning device range. Designing the load modulation network with high dynamic range will severely deteriorate the bandwidth for a Pi shaped load modulation network. A T-shaped network has a superior bandwidth under all circumstances, compared with a Pi shaped network.
The bandwidth of the load modulation network can be substantially increased by replacing the Pi shaped network with a T shaped network. The names Pi shaped network and T shaped network refer to the shapes of the respective load modulation networks.
However, the T shaped network has a relation of network input impedance to tuning device capacitance inverse to that of the Pi network. This inverse relation of input impedance to tuning device capacitance creates some major drawbacks using a T shaped network. Some examples of such major drawbacks are very high power dissipation in the tuning device and a very limited dynamic range, a few decibels (dB). The high power dissipation is due to high values of Radio Frequency (RF) voltage at the tuning element node simultaneous as large values of capacitance. The limitation in dynamic range is due to the same mechanism. The bias voltage to the tuning device can not be set to lower values, i.e. large capacitance values since this would give a reverse voltage over the tuning device by the peaks of the Radio Frequency voltage. This could potentially destroy the tuning device, or give rise to severe distortion.